The default gateway is not available

I just bought a Sony VAIO VPCEA3S1E and I have a problem with my WIFI connection. It says it connects to the local network and internet, but in reality, internet doesn't work. When I try to 'resolve the problem', it indicates that "The default gateway is not available". I've updated all things I could update (including VAIO related, Windows update, as well as the driver of the WIFI card) but the problem remains. In contrast, my flatmates manage to connect to the Wifi network. Also, I can have internet through the ethernet cable.

FYI, my Wifi card is ATHEROS AR9285 Wireless Network Adapter. I'm also attaching a zipped version of the information system (.rar).

I thank anyone who could tell me how to resolve this issue, as I really need to access internet through the Wifi...